Such electronically commutated DC motors (BLDC motors or EC motors) are generally known and comprise as the rotor a permanent magnet, for example, which is driven by a rotating excitation field. This excitation field is produced, for example, by a three-phase winding system configured in a star-connection or delta-connection using trapezoidal or sinusoidal current waveforms, which are phase-offset with respect to one another, to energise the winding phases thereof.
Commutation of a BLDC motor is typically performed on the basis of microprocessor- or software-based open-loop or closed-loop control of the individual phase currents of the windings of the winding system of the BLDC motor by using in a known manner, for instance, a triple half-bridge consisting of power switches, for example MOSFETs, to generate a plurality of currents through the winding system that differ in phase position and amplitude. The power semiconductors are controlled by a microprocessor, which needs to know the rotor position of the rotor in order to determine the optimum commutation times. The rotor position can be ascertained without sensors or using an additional sensor system.
Various methods are known for ascertaining without sensors the rotor position of the rotor of a BLDC motor. In a first group of methods, the current rotor position is ascertained by analysing the zero crossovers of the induced back voltage (EMF) in the winding phases that are not currently energised, because a voltage vector induced in the winding system is uniquely associated with the rotor position. This analysis assumes that the rotor is stationary, however.
Although such a method can also be used when the rotor is rotating very slowly, the errors in the ascertained rotor position increase as the rotational speed increases.
A second group of methods is based on the variation of the inductance of the BLDC motor. As a permanent magnet, the rotor produces a magnetic asymmetry because the reluctance is greater in the direction of the magnetisation of the rotor (d-axis) than in the transverse direction (q-axis). This results in an inductance of the BLDC motor that is dependent on the rotor position. That stator winding phase for which the magnetic axis is coincident with the d-axis of the rotor has a minimum inductance, and that winding phase for which the magnetic axis is coincident with the q-axis, i.e. rotated through 90°, has a maximum inductance. In the non-energised state of the BLDC motor, the south pole and the north pole of the rotor as a permanent magnet have the same effect and hence the characteristic of this variable inductance has twice the periodicity of the electrical variables. This 180° ambiguity must be resolved for complete position information. To do this, a voltage signal is applied according to the direction of the rotor so as to reduce or raise the saturation in the stator, i.e. decrease or increase the corresponding inductance, in order to be able to determine the rotor position therefrom.
Document AT 395 487 B, for example, discloses this inductive approach to determining the rotor position, in which the current pulses, generated by voltage pulses, and associated voltage pulses are detected and the resultant inductances are determined. These inductance values are assigned to a sinusoidal characteristic around the circumference in order to ascertain therefrom the current position within the sinusoidal characteristic. According to this known method, a second measurement must be performed in order to be able to compensate for the EMF voltage arising in the measurement result when the rotor is rotating.